I vote for the U60, have two myself, but no SB150, so I can't really compare, but ... the U60's cpus have 4MB cache, the SB150 only 512KB. And you have two UPA-slots. And SCSI. And place for two disks. And it's much cooler ...

Depends a bit on what you want it for I guess but for me I'd definitely take the Ultra 60. For a start the difference in ram probably accounts for most of the difference in price! The ide in the Blade is ata66 I think - pretty dog slow anyway - but at least you can throw a cheap 120gb disk in it if you need to boost storage. The Blade 150 is basically a Blade 100 with a faster (slightly) processor. Its very much at the budget workstation end of the line.
The Ultra 60 is a classier machine and those dual cpus with 4mb cache bad boys should give you some decent throughput.
On the software side if you are looking to run Gentoo some people have had problems with Blade series machines you may want to search these forums to aquaint yourself with any potential hitches. If you are looking to try out Solaris you may find that a dual processor machine doesnt qualify for a free license (this may have changed with Solaris10 but its worth checking out).

If you want to run gentoo, go for the U60 .. the system overall will be a bit faster, and your hardware will be well supported mostly.
With the SB150 you'll have a slightly slower system, and your onboard hardware will be a tad less supported. Also, the onboard IDE in the SB150 sucks seriously (think ultra5 style)...

However, if you want to go solaris, and play with all the latest & greatest sun addons (sunpci IIIpro, etc) or mess around with the solaris10 goodness, go with the sb150 .. it WILL be a tad slower, however the hardware is still in a "supported" stage from Sun and new hardware addons continue to support it.

Additionally, whichever you decide to get, can you let me know where I can get the one you didnt want? Those are some CHEAP prices _________________-Tim Smith